The present invention relates to creating traps in the context of digital prepress.
Press registration is the accurate positioning of two or more colors of ink on a printed sheet. When the colors on a sheet are in register, all colors appear precisely where intended, without unintended gaps between colors or overlap of colors. Misregistration can be caused by a number of factors. One result of misregistration is unprinted paper showing through between colors, where no unprinted area was intended. Even a small registration error can result in a thin white line showing between colors on a sheet.
Trapping is the process of compensating for press misregistration by intentionally overlapping colors prior to printing. Trapping involves deciding where to place traps, and setting the size and color of traps so as to correct for possible misregistration in a way that is minimally noticeable to the human eye. Traps are generally small objects in the form of polygons. A trap is constructed by defining a rectangle along an image edge and then shaping the rectangle at its ends and along its length to conform to nearby traps and objects. The width of the initial rectangle is referred to as the trap width.
It is conventional in the electronic publishing industry to design pages using a page description language (PDL), such as the Adobe® PostScript® language, to describe a page using resolution-independent elements. (Adobe and PostScript are trademarks of Adobe Systems Incorporated of San Jose, Calif.) A PDL description of a page may be analyzed to produce a new PDL description which contains traps. The new page description may then be used to render the page, with traps, according to conventional methods. Alternatively, a page may first be rasterized and the traps may then be generated in raster form by analyzing the rasterized page. Both vector-based trapping and raster-based trapping are done automatically in prior art systems. (In this context of page description language, a vector description is one based on resolution-independent graphics primitives and is not limited to straight-line vectors.)
FIG. 4 illustrates the concepts of trapping pertinent to this specification. Two objects are to be printed over each other with different colorants: a small square 402 and a large square 404. The large square 404 has a knockout 406 for the small square. If the small square is of exactly the same size as the knockout and no misregistration occurs in printing, the small square prints within the large square with no gaps 408. If any misregistration occurs, visible substrate 410 shows through as a result of misregistration. A conventional trapping process will spread the large square 404 into the small square 402, or spread the small square 402 into the large square 404, so that the appearance of the printed page will be acceptable even when misregistration occurs.
Digital publishing systems generally provide trapping subsystems, which will be referred to as trap engines, that automatically generate traps. Traps are generated at edges between regions of different color. Conventional trap engines generate traps having symmetric trap widths—that is, the perpendicular distance a trap extends from an edge is calculated without reference to the orientation of the edge on the printed page. The trap width is selected to compensate for a maximum acceptable misregistration in the printing process for the page.
As was mentioned, some trap engines are raster-based. Raster-based trap engines generate traps, in raster or vector form, depending on the trap engine, for raster objects. As do vector-based trap engines, raster-based trap engines generate symmetric traps. Raster-based trap engines do so in a way that assumes that the raster, i.e., the printer resolution, is symmetric—that is, that the raster aspect ratio is 1:1.
One prior art Adobe® vector-based trap engine handles a printer with an asymmetric aspect ratio as follows. The engine asymmetrically converts the vector page description data into trap engine space, generates traps, and asymmetrically converts the results back to conform to the aspect ratio of the printer. However, this prior art has no ability to generate asymmetric traps to compensate for systematic asymmetric misregistration in the operation of a printing system.